Male-to-female sexual transmission of human immunodeficiency virus type 1 (HIV-1) infection is approximately twice as efficient as the opposite. Progress has been made in defining the initial steps of HIV-1 pathogenesis in women. A landmark study released in 2010 showed proof of concept that a vaginal gel containing 1% tenofovir was effective in preventing male-to-female transmission in 39-54% of women using the gel peri-coitally. However, major gaps exist in understanding HIV-1 acquisition in women and in predicting the success of worldwide implementation of HIV-1 prevention products. Specifically, the influence of female reproductive hormones, whether during the menstrual cycle or in menopause, on HIV-1 acquisition and transmission remains to be clearly elucidated? All women will naturally experience fluctuations of reproductive hormones throughout their life. Animal data and information from small cohorts of women suggest that women may be more susceptible to HIV-1 infection in estrogen-deplete or progesterone-dominant states, such as menopause and luteal phase of the menstrual cycle. No human data exist on the effects of the female reproductive cycle or hormonal therapy on the pharmacokinetics and pharmacodynamics of tenofovir vaginal gel. Our studies will fill major gaps in our knowledge about HIV-1 infection and prevention. Primarily, we will elucidate the effect of women's reproductive cycles on HIV-1 susceptibility and tenofovir pharmacokinetics and pharmacodynamics at the level of the cervicovaginal mucosa. We will characterize these parameters through innovative experiments which include obtaining ectocervical tissue explants and biopsies from pre and post menopausal women. In order to better control for potential confounding variables, we will obtain genital tract biopsies in a longitudinal clinical study of premenopausal women in the follicular and luteal phases, both at baseline and after exposure to vaginal tenofovir gel. Similarly, we will obtain genital tract biopsies from postmenopausal women at baseline and after exposure to tenofovir vaginal gel. Through these experiments, we will test the hypotheses that progesterone dominant states (high progresterone/estrogen ratio), such as the luteal phase of menses and menopause, increase cervicovaginal susceptibility to HIV-1 and alter tenofovir pharmacokinetics and pharmacodynamics.